This invention relates to Cr2O3xe2x80x94Al2O3 ceramics which can be expected to be applicable to high-temperature components subject to severe corrosion by molten salts, such as coatings for the rotor blades of low-quality oil-fired gas turbines, burner nozzles and diffuser cones of crude fuel-fired boilers, and stokers of refuse incinerators, and to processes for producing the same.
The resistance of Cr2O3xe2x80x94Al2O3 ceramics to corrosion by molten salts comprising heavy metal oxides and alkali salts depends primarily on the sintering temperature and the content of Cr2O3xe2x80x94Al2O3 solid solution. In order to achieve high corrosion resistance, it is necessary to form a dense sintered body consisting of a complete solid solution of Cr2O3 and Al2O3. However, since Cr2O3 has poor sinterability, a hot isostatic pressing machine or the like must be used to yield a dense sintered body.
Consequently, conventional processes have the disadvantage that they require a considerable cost and have a limited throughput owing to the use of a hot isostatic pressing machine or the like. Moreover, the conventional processes also have the disadvantage that the formation of a Cr2O3xe2x80x94Al2O3 solid solution does not proceed to the fullest extent, causing Cr2O3 and Al2O3 particles to remain in the sintered body.
The present inventors made intensive investigations on processes in which TiO2 serving as an assistant for accelerating the diffusion of Al2O3 and Cr2O3 is added in order to improve the sintered density and promote the formation of a Cr2O3xe2x80x94Al2O3 solid solution, and in which finely powdered raw materials are sintered under conditions including a suitable atmosphere and a suitable temperature. As a result, it has been found that, by adding TiO2, using finely powdered raw materials, and employing a suitable sintering atmosphere and a suitable sintering temperature, an improvement in sinterability can be achieved to yield a sintered body showing an improvement in compositional homogeneity and resistance to corrosion by molten salts. The constitution of the present invention is as follows.
A process for the production of a sintered body having excellent resistance to corrosion by molten salts which comprises the steps of providing a powder mixture A consisting essentially of Cr2O3 powder having an average particle diameter of not greater than 0.5 xcexcm and Al2O3 powder having an average particle diameter of not greater than 0.5 xcexcm, mixing the powder mixture A with TiO2 powder having an average particle diameter of not greater than 0.5 xcexcm to form a powder mixture B containing the TiO2 powder in an amount of 1.5 to 15% by weight based on the combined amount of the powder mixture A and the TiO2 powder, and sintering the powder mixture B at a temperature of 1,350 to 1,600xc2x0 C. in a vacuum atmosphere of 10xe2x88x9220 to 10xe2x88x925 atm. to yield a sintered body consisting of an intimate solid solution of Cr2O3 and Al2O3 and having a relative density of not less than 95%.
A process for the production of a sintered body having excellent resistance to corrosion by molten salts which comprises the steps of providing a powder mixture A consisting essentially of Cr2O3 powder having an average particle diameter of not greater than 0.5 xcexcm and Al2O3 powder having an average particle diameter of not greater than 0.5 xcexcm, and sintering the powder mixture A at a temperature of 1,350 to 1,600xc2x0 C. in a vacuum atmosphere of 10xe2x88x9220 to 10xe2x88x925 atm. to yield a sintered body consisting of an intimate solid solution of Cr2O3 and Al2O3 and having a relative density of not less than 95%.
A process for the production of a sintered body as described above in embodiment 2 wherein the sintering temperature is in the range of 1,450 to 1,600xc2x0 C.
A process for the production of a sintered body as described above in any of embodiments 1 to 3 wherein the Cr2O3 powder is used in an amount of 40 to 60% by weight based on the powder mixture of Cr2O3 powder and Al2O3 powder.
A sintered body having excellent resistance to corrosion by molten salts which is a sintered body consisting essentially of a solid solution phase composed of Cr2O3 and Al2O3 and having a relative density of not less than 95% and which contains no phase composed of Al2O3 alone.
The above-described processes for the production of a sintered body in accordance with the present invention make it possible to produce a dense sintered body consisting of a homogeneous solid solution of Cr2O3 and Al2O3 and thereby achieve a concomitant improvement in corrosion resistance.